Observational Astronomy

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Observational Astronomy

• TELESCOPES,
• Active and adaptive optics
• Kitchin pp.51-129

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Optical Schemes

• Spherical mirrors cannot focus light properly
  due to spherical aberrations

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Optical Schemes

• but a single parabola can produce perfect
  image on the optical axisOff-axis images
  suffer from coma.

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Ritchey-Chrétien telescope

• Hyperbolic primary and hyperbolic secondary
  solve main aberration problems (sphericaland
  coma) in a rather large field of view(tens of
  arcminutes) in Cassegrain focus

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Schmidt-Cassegrain

• RC provides very good image quality in a
  relatively small field (1º - 2º)
• When large FoV (up to 5º) is need
  Schmidt-Cassegrain is the preferred design

Correction plate
Spherical mirror
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Alternative to RC Gregorian system

• Concave secondary after the primary focus

Courtesy of AIP Potsdam
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Materials

• Low thermal expansion zerodur sitall

mean linear coefficient of thermal expansion
within temperature range-60º to 60º ? is lt10-8
cm ºC-1
Astro-sitall blank at LZOS(VST, VISTA, SALT,
LAMOST, OWL?)
Zerodur VLT primary at REOSC
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More materials

• Silicon Carbide
• Low thermal expansion(not as good as glass)
• Very light
• Very hard, keeps the shape well
• Hard to make in large pieces
• Fragile, difficult to process

SiC 60 cm X-ray mirrorWeight 6.2 kg
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Coatings

• Mirrors
• Aluminum (forming SiO) on the top
• Silver-based coatings. Needs coating to prevent
  mechanical damage during washing
• Lenses MgF2

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Reflectivity
Reflectivity
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Point Spread Function

• PSF is the intensity distribution in the focal
  plane produced by a point source.
• Ideal PSF (Bessel function)

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Active optics

• Large thin mirrors are shaped bysupport system
  VLT mirror is8.2m in diameterand only 18 cm
  thick!

• Compensate for thermal and orientation
  distortions
• Close loop operation during adjustment
• Low frequency30 s cycle
• VLT 150 actuators

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Adaptive optics

• Seeing corrections (PSF)The Strehl ratio is
  the ratio of peak intensities in the aberrated
  and ideal point spread functions in the focal
  plane (Born and Wolf 1999).

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Why do we need adaptive optics?

• Atmospheric turbulence distorts the wave front.

• Three ways of looking at the focal plane image
• Non-collimated beams (speckles)
• Curved wavefront (phase shifts)
• Changing intensity distribution

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Wavefront sensor

• Shack-Hartmann
• Curvature sensor
• Pyramid WFS

Fast steering mirror is neededto get all pixels
in focus
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Sensor implementation
Wavefronts must measured many at 100 kHz rate!
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Deformable mirror
Various shapes of a deformable mirror with 37
actuators
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Calibrations

• Looking for a zero-point of DM

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Closing the loop
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Laser Guide Star
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Final result

• VLT NACOPSF and resolutionimprovements

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Home work

• Find a telescope with Silicon Carbide mirror. Why
  this material was used?
• When using adaptive optics what are the pluses
  and minuses of using natural and laser guide star?

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Next time

• Direct Imaging and Photometry